The wireless mobile telephone system of a cellular system includes a plurality of wireless base stations each arranged and fixed at each of cell regions which are produced by dividing an area over which a telephonic communication service is available into a plurality of regions, and mobile stations which are mounted in a moving body such as an automobile and communicate with the wireless base station, wherein several of the wireless base stations produce a group.
In such a prior art wireless mobile telephone system, the cell region has conventionally, for example, a size of 1-10 km, but recently a micro cell system is proposed in which the size of the cell is small as, for example, 500 m. This system has an effect that low transmission powers are sufficient for the mobile station and for the base stations due to the small size of the cell region while the fundamental construction of the system is the same as the usual cell system. In addition, a miniature sized battery is sufficient for the mobile station and thereby the apparatus is minimized. The lifetime of the battery for the mobile station is also lengthened, and a long time telephonic communication is possible. Further, it is possible to offer a telephonic communication service even to an area where no service is available due to being beyond the reach of electric wave in a large zone system, for example, by employing a wall hanging type base station in a building, in an underground town, in a tunnel of a subway, or the like.
In many of such prior art wireless mobile telephone systems, because a fixed channel is assigned to each wireless zone, there may be an idle channel in a peripheral zone even when some wireless zone has no idle channel. In addition, differently from in wire communication systems, boundaries between the wireless zones are not clear and usually overlapping areas are produced where the mobile station can be connected to any of the base stations of the wireless zones which own the overlapping zones jointly.
Accordingly, for a mobile station existing in this overlapping channel it is possible to perform a so-called other zone selection connecting a mobile station employing an idle channel of the other zone even when there is no idle channel in some wireless zone.
Such other zone selection is particularly effective in a micro cell system. It is because, while in a large zone system the base station can hold a lot of channels, i.e., sixteen to ninety channels, in the micro cell system the base station can hold only quite a small number of channels, for example, four channels, and also because in this micro cell system, since the ratio of the overlapping area occupying the area of the cell is large relative to the large zone system, it is possible to reduce the origination loss, i.e., the states of waiting telephonic communication, when the mobile station is present in the overlapping area and the base station has no idle channel by assigning a channel to the other base station producing the overlapping area.
FIG. 12 is a diagram illustrating a construction of a wireless mobile telephone system performing such other system selection according to a prior art. The system construction is disclosed in FIG. 5 as a prior art system construction of other zone selection system in Japanese Published Patent Application Hei. 4-180314. FIG. 13 is a diagram illustrating a sequence of this other zone selection disclosed in FIG. 6 of the Published Application.
FIGS. 17 to 20 are diagrams illustrating signal formats of the control channel and the voice channel in such a system. FIG. 17 shows a transmission message in the control channel from the mobile station to the base station, FIG. 18 shows a transmission message in the voice channel from the mobile station to the base station, FIG. 19 shows a transmission message in the control channel from the base station to the mobile station, and FIG. 20 shows a transmission message in the voice channel from the base station to the mobile station.
In FIGS. 12 and 13, reference numeral 1 designates a mobile station of such as an automobile telephone or a portable telephone. Numerals 2, 3, and 4 designate base stations performing a wireless telephonic communication with the mobile station 1. Numeral 5 designates a base station controller performing a line control of respective base stations. Reference numeral 6 designates an origination signal which the mobile station 1 transmits to the base stations 2, 3, and 4. Numerals 7, 8, 9, 10, and 11 designate inter-terminal signals which are transferred between the base stations 2, 3, and 4 and the base station controller 5 for assigning a voice channel to the mobile station 1. Numerals 12, 13, and 14 designate wireless zones over which the base stations 2, 3, and 4 offer telephonic communication services, respectively. Numeral 15 designates an overlapping zone of the wireless zones 12, 13, and 14. It is supposed that the mobile station 1 is present in this overlapping zone 15.
In FIGS. 17, 18, 19, and 20, reference numerals 301, 310, 313, 316, 319, 322, 325, 331, 340, 350, 353, 356, 359, and 362 designate bit synchronization patterns for performing bit synchronization, and reference numerals 302, 311, 314, 317, 320, 323, 326, and 332 designate word synchronization patterns for performing word synchronization. Reference numeral 303 designates a coded digital color code for distinguishing a base station to be accessed when interferences occur at the same frequency. Reference numerals 304 to 306 designate a first, a second, and a third word which are respectively a repetition of words, and one message of origination includes one or a plurality of words and each word is transmitted five times. Reference numeral 312 designates the first word in a first time repetition, numeral 315 designates the first word in a second time repetition, numeral 318 designates the first word in a third time repetition, numeral 321 designates the first word in a fourth time repetition, and numeral 324 designates the first word in a fifth time repetition. Reference numeral 327 designates the second word in a first time repetition and numeral 328 designates the second word in a fifth time repetition. Reference numerals 330, 339 designate a busy/idle bit indicating whether the apparatus is in an operating state or not. Numeral 333 designates word A in a first time repetition, numeral 334 designates word B in a first time repetition, numeral 335 designates the word A in a second time repetition, numeral 336 designates the word B in a fourth time repetition, numeral 337 designates the word A in a fifth time repetition, and numeral 338 designates the word B in a fifth time repetition. Reference numeral 352 designates a word in a first time repetition, reference numeral 355 designates the word in a second time repetition, reference numeral 358 designates the word in a ninth time repetition, numeral 361 designates the word in a tenth time repetition, and numeral 364 designates the word in an eleventh time repetition.
A description is given of the operation with reference to FIGS. 12 and 13. When the mobile station 1 calls in the overlapping zone 15, the origination signal 6 is transmitted through the up control (access) channel common to the zones, and received by the base stations 2, 3, and 4. Respective base stations 2, 3, and 4 add information indicating the receiving level of the origination signal 6 and the presence of an idle channel to the received origination signal 6 and transmit the same to the base station controller 5 as calling signals 7, 8, and 9, respectively.
The base station controller 5 investigates the calling signals 7, 8, and 9 and selects a base station having an idle channel and the maximum receiving level among them. Suppose that the base station 3 is selected in this example, the base station controller 5 transmits a channel request signal 10 to the base station 3. The base station 3 which has received the channel request signal 10 selects a channel and reports the selected channel by a channel reporting signal 11 to the base station controller 5. By the above-described sequence, selection of the other zone is realized.
In the prior art wireless mobile telephone communication method, the selection of the other zone is realized by the above-described sequence. However, because in this system the base station having an idle channel and the maximum receiving level is selected, the base station having a less number of idle channels but the maximum receiving level is priorly selected, whereby in the base station in which the traffics are concentrated, it is still likely to occur origination failure.
In addition, since in this prior art system it is presumed that an access control channel that is common to zones is employed, the selection of other zone is impossible in the system assigning individual control channels to respective base stations.
Further, since the base station controller 5 judges channel selection and other zone selection when all the calling signals 7, 8, and 9 reach the base station controller 5, processing burden to the base station controller 5 is large.
The problem that the processing burden to the base station controller 5 is large is solved by an apparatus disclosed in FIGS. 2 to 4 of the above-described Japanese Published Patent Application Hei. 4-180314 by dispersing the control of the channel selection. In this apparatus, the mobile station observes peripheral zones so as to select wireless zones which become candidates in the other zone selection, and the wireless base station performs assignment of a wireless channel considering all wireless zones as the candidates, thereby effectively utilizing the frequency.
FIGS. 14 to 16 show those disclosed in FIGS. 2 to 4 of the above-described published application.
In FIG. 14, it is assumed that the wireless zone 225 produced by the wireless base station 223 and the wireless zone 226 produced by the wireless base station 224 overlap with each other and the mobile station 222 is present in this overlapping zone. The data line 227 connects the wireless base station 223 and the wireless base station 224. The wireless communication channel 220 is a channel between the base station 224 and the mobile station 222, and the channels 228 and 229 are observing channels for observing the base stations 223, 224 and a wireless control channel transferring information during waiting. The wire transmission line 231 connects the wireless base station 224 and the fixed terminal 232. The fixed terminal 232 communicates with the mobile station.
A description is given of an operation from waiting a communication to assigning a wireless channel in response to a communication connection request with reference to FIG. 15.
First of all, during waiting a communication, the mobile station 222 judges that it is present in the wireless zone 225, and receives the control channel 228 from the wireless base station 223. The wireless base station 223 communicates the frequency of the observed channel in the peripheral zone to be observed during a period not receiving the control information, via the control channel 228.
In this second prior system, the frequency of the control channel 228 of the base station 224 of the peripheral zone is communicated through the control channel 228 to the mobile station 222, and the mobile station 222 switches its frequency to the communicated frequency circulatingly, and measures the receiving level of the overlapping base station 224 itself. When the receiving level of the observed channel of the peripheral zone exceeds the receiving level of the control channel of the zone where the mobile station is present, the mobile station judges that it moved to the other zone and it switches the control channel which it receives, but it constantly measures the channels to be observed of the plurality of peripheral zones communicated through the control channel even after the movement to the other zone.
When a communication connection is requested (233) by occurrence of calling and receiving of calling by the mobile station 222, the mobile station 222 reports the receiving level of the observed channel 229 in the wireless zone 226 as the above-described peripheral zone to the base station 223 using the control channel 228 of the existing zone 225. The base station 223 of the existing zone 225 performs a selection of a wireless channel in response to a communication origination, but if there is no available channel in the base station 223, the base station 223 judges whether the receiving level of the channel observed reported from the mobile station 222 is larger than a predetermined level and any other base station is possible to be connected or not.
In this second prior art system, the mobile station 222 is present in the overlapping area of the wireless zone 225 and the wireless zone 226, the receiving level of the channel observed becomes the maximum, it is judged that the base station 224 is possible to be connected, and the base station 223 inquires 234 the base station 224 through the date line 227 whether there exists an available wireless channel or not in the base station 224.
When there are a plurality of base stations as candidates for base stations that are possible to be connected, an inquire is continued (234) until an available wireless channel is found in the order from a larger receiving level, and the wireless channel which has the maximum receiving level is selected.
When there is a channel response 235 that there is an available channel in the base station 224, a CH setting 236 designating the frequency of the wireless communication channel 220 the mobile station 222 is performed, and a communication path 220 with the fixed terminal 232 is set through the wire transmission path 231.
FIG. 16 show a flowchart for setting a communication path of the above-described system. In a communication waiting state or in a communicating state (step 240), the mobile station 222 is communicated with the frequency of the observed channel of the peripheral zone from the base station 223 through the control channel 228 (step 241). Thereby, the mobile station 222 switches the frequency to that communicated from the base station 223 circulatingly each constant time circulatingly (step 242), and it measures the receiving level of the frequency received from the observed channel (step 243). The receiving level measured of the observed channel and the receiving level of the control channel of the zone where the mobile station 222 is present are compared and when the receiving level of the observed channel exceeds the receiving level of the existing zone (step 244), the mobile station 222 switches the control channel and moves from the wireless zone 225 to the peripheral zone 226 (step 45). After moving the zone, the mobile station 222 measures the receiving level of the observed channel of the plurality of peripheral zones which are communicated through the control channel (step 246).
When a communication connection is requested or a switching of a channel is requested by an origination and receiving origination of the mobile station (step 247), when the mobile station 222 is present in the wireless zone 225, the mobile station 222 reports the receiving level of the observed channel of the wireless zone 226 as one of the peripheral zones using the control channel 228 of the existing-in-zone 225 to the base station 223 (step 248). The base station 223 of the wireless zone 225 where the mobile station 222 is present investigates whether there is an available channel in the base station 223 or not when selecting a wireless channel in response to a communication connection request (step 249), and selects a wireless channel (step 253) and sets a communication path when there is an available channel (step 254).
On the other hand, when there is no available channel with traffics concentrated, the base station 223 compares the receiving level of the observed channel that is reported from the mobile station 222 at step 248 with a prescribed threshold (step 250), and when the receiving level is higher than the threshold value, it investigates whether there is an available wireless channel in the other zone 226 (the above-described observed channel) or not (step 251). When there is an available channel 220 in the other zone 226, the base station 224 designates the frequency of the channel 220 to the mobile station 222 to set a channel (step 252), and sets a communication path with the fixed terminal 232 through the wire transmission path 231.
In the second prior art system illustrated in FIGS. 14 to 16, even in a case where traffics are concentrated in the wireless zone of the base station 223 and there is no channel for setting in the existing mobile station 222, if there is a channel available for the overlapping other zone 226, the other base station 224 can set a channel in the mobile station 222 and set a communication path, whereby it is possible to respond to the origination and receiving origination by the mobile station 222.
In addition, since wireless zones which become candidates in other zone selection are selected by the above-described observation of peripheral zones of the mobile station and assignment of wireless channels is performed considering all the wireless zones as candidates in the wireless base station, it is possible to enhance the utilization efficiency of the frequency by the processing diversified to the respective base stations.
In this second prior art system, however, when there are a plurality of candidate base stations that are possible to be connected, a wireless channel having the maximum receiving level among them is selected and, therefore, even a base station having a less number of idle channels, it is selected on priority basis if it has the maximum receiving level, and the base station in which traffics are concentrated is still likely to occur calling loss.